Genetics: Dihybrid Crosses

Biology 12

Joke of the day…

What is a dihybrid cross?• These are crosses that involve genes for two

different characteristics

Back to Mendel and his peas…

Mendel: Dihybrid Crosses• The natural progression for Mendel was to study

two characteristics at the same time.

• Thus, the study of 2 pairs of contrasting traits at the same time = a dihybrid cross

• ex.. round yellow seeds X wrinkled green seeds

Review: Mendel:• Principle of Independent Assortment• When more then one trait is studied in the same cross,

the genes for each trait sort into gametes independently of the genes of the other traits

Mendel’s peas:

Character Trait AlleleSeed shape Round A

Wrinkled aSeed colour Yellow B

Green b

Let’s cross a homozygous round/yellow (AABB) with a homozygous wrinkled/green (aabb) plant….

Steps for Dihybrid Crosses:1. Determine Genotypes of Parents2. Determine Genotypes of Gametes

(remember gametes only have 1 copy of each gene) and think FOIL (from math class!)

3. Punnett Square: need 16 squares now, 4 gametes from each parent

4. Identify Phenotype Ratios in Offspring

Example:o In snapdragons…

o Tallness (T) is dominant to dwarfness(t)o Red color is due to gene (R) and white to its

corresponding allele (r). o The heterozygous condition results in pink (Rr)

flower color.

o A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. Give the possible genotypes and corresponding phenotypes for all of the possible F1 generation.

Step #1: Determine Genotypes of Parents

“A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. “

Parent 1:Dwarf pink = ttRrParent 2:Homozygous tall, red

= TTRR

Step #2: Determine Genotypes of Gametes

Dwarf pink = ttRr

Homozygous tall, red = TTRR

tR tr tR tr

TRTR TR TR

TR TR TR TR

Gametes from TALL red parent

tR

tr

tR

tr

Gametes from dwarf pink parent

Step #3: Punnett SquareThe Punnett square determines the genotypes of the offspring

TtRR

TtRr TtRr

TtRR

TtRr

TtRR TtRR

TtRrTtRr TtRr

TtRR TtRR

TtRr

TtRr

TtRR

TtRR

Step #4: Identify Phenotype Ratios in Offspring

Tall, red = 8/16Tall, pink = 8/16

Tall, white = 0/16Short, red = 0/16

Short, pink = 0/16Short, white = 0/16

Try it :• Page 154 questions 1, 3 (and then try #2)